Known in the state of the art are so-called oscillatory forks and also so-called single-rods, with which, for example, the fill-level of a medium is measurable. For such purpose, the oscillatory fork or the single-rod is excited to execute mechanical oscillations. The characterizing variables of the oscillations, amplitude, frequency and phase, depend on, among other things, whether the oscillatable unit—in the case of the fork, the two fork tines—are oscillating freely or covered by the medium. Thus, for example, in the case of liquids, the frequency falls, when the fork or the single-rod transitions from the uncovered state to the covered state. Therefore, it is possible, by evaluating frequency, to deduce the fill-level. Often, such measuring devices are used for limit-level monitoring. In such case, one distinguishes between monitoring a lower (min-protection) and an upper (max-protection), limit level. The pertinent limit-level depends, in such case, on the installation height and the dimensioning of the measuring device and especially the dimensioning of the oscillatable unit. The oscillatable unit is, most often, secured on a membrane, or diaphragm, behind which the driving/receiving unit is located. The driving/receiving unit serves for exciting the oscillatable unit to execute oscillations and also for receiving the oscillations. The driving/receiving unit is often a piezoelectric element, which changes an electrical, alternating voltage into a mechanical oscillation, or, in reverse, mechanical oscillations into an alternating voltage. Evaluation of the alternating voltage permits, then, the ascertaining of the characterizing variables of the oscillation. In an embodiment, such as described, for example, in EP 1 134 038 A1, the driving/receiving unit is a stack of piezoelectric elements, which is secured between a hemisphere and a disk. This stack is pressed against the membrane, or diaphragm, under a certain prestress, in order to achieve an optimal force transfer.
A problem for this construction arises in the case of high temperature applications. If a temperature jump occurs, then, due to the different coefficients of expansion of the membrane, or the housing, of the measuring device and the driving/receiving unit, and due to the time for uniform spreading of the temperature in the measuring device, it can happen, that, for a certain time, contact is lost between driving/receiving unit and the membrane, or diaphragm. This, of course, means that the measurements worsen or even become impossible. This is intolerable, especially for safety-relevant, fill-level monitoring.